The Node.lookupPrefix() method returns a DOMString containing the prefix for a given namespace URI, if present, and null if not. When multiple prefixes are possible, the result is implementation-dependent.
Due to bug 312019, this method does not work with dynamically assigned namespaces, that is those set with the Node.prefix property.
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https://developer.mozilla.org/en-US/docs/Web/API/Node/lookupPrefix